1. Field of the Invention
This invention relates to a magnetic field sensor comprising Wiegand wires or similar wirelike bistable magnetic elements.
2. Description of the Prior Art
Wiegand wires are twisted ferromagnetic wires which have a homogeneous composition and consist, e.g., of an iron-nickel alloy comprising preferably 48% iron and 52% nickel, or of a cobalt-iron-vanadium alloy comprising preferably 52% cobalt, 38% iron and 10% vanadium, and which have been subjected to a special mechanical and thermal treatment resulting in the formation of said wires with a soft magnetic core and a hard magnetic shell, which has a higher coercive force than the core. Typical Wiegand wires have a length of 10 to 50 mm, preferably of 20 to 30 mm. When a Wiegand wire which has been magnetized to saturation in a magnetic field having a magnetic field strength of at least 80 A/cm and preferably in excess of 100 A/cm so that the soft magnetic core and the hard magnetic shell are magnetized in the same direction and said Wiegand wire is introduced into an external magnetic field which has the same direction as the axis of the wire and said direction is opposite to the direction of flux in the Wiegand wire, the direction of flux in the soft core of the Wiegand wire will be reversed when the magnetic field strength of said external magnetic field exceeds a value of about 16 A/cm.
That reversal can also be called "resetting" and the magnetic field strength required for that reversal is called "resetting field strength".
When the external magnetic field strength is reversed once more beyond a critical field strength, which is about 8 to 10 A/cm below the resetting field strength and is called "triggering field strength", the direction of flux in the core will be reversed again so that the core and the shell will again have the same directions of flux. That phenomenon is called the "triggering" of the Wiegand wire. That reversal of the direction of flux takes place very quickly and is accompanied by a correspondingly large change of the magnetic flux per unit of time (Wiegand effect). That change of the magnetic flux can induce a short and very strong voltage pulse (Wiegand pulse) of up to about 12 volts in an induction winding, which is called detector winding and the number of turns and the load resistance of which will determine the induced voltage. Wiegand pulses have a duration of about 20 microseconds. As a rule, the detector winding has between 1000 and 4000 turns, in most cases about 2000 turns, and a diameter below 3 mm, in most cases between 2.0 and 2.5 mm. If the Wiegand wire lies in a magnetic field which is reversed from time to time and which is so strong that it can reverse the magnetization first of the core (at the lower triggering field strength) and subsequently also of the shell (at a higher field strength) and can effect saturation in each case, the reversal of the direction of flux in the soft magnetic core will result in the generation of Wiegand pulses of positive and negative polarities in alternation. This is described as a symmetrical excitation of the Wiegand wire. For this purpose field strengths from about -(80 to 120 A/cm) to +(80 to 120 A/cm) called "saturating field strengths", are required. The reversal of the direction of flux in the shell takes also suddenly and results in a pulse in the detector winding but that pulse is much smaller than the pulse induced by the preceding reversal of the direction of flux in the core.
If the selected external magnetic field can reverse only the direction of flux in the soft core but cannot reverse the direction of flux in the hard shell, the strong Wiegand pulses which are generated will have only one polarity. This result is described as an asymmetric excitation of the Wiegand wire. For this purpose a field strength of at least 16 A/cm is required in one direction (for resetting the Wiegand wire) and a field strength of about 80 to 120 A/cm is requried in the opposite direction.
It is typical of the Wiegand effect that the amplitude and width of the pulses which are generated by it are substantially independent of the rate of change of the external magnetic field and that they have a high signal-to-noise ratio.
The invention is also applicable to other bistable magnetic elements, which have two magnetically coupled regions which differ in hardness (coercive force), and like Wiegand wires can be used to generate pulses by a quick reversal of the direction of flux throughout the soft magnetic region by a large Barkhausen jump. For instance, German patent specification No. 25 14 131 discloses a bistable magnetic switching core in the form of a wire having a hard magnetic core, e.g., of a nickel-cobalt alloy, an electrically conductive intermediate layer, e.g., of copper, deposited on the core, and a surface layer, which has been deposited on said intermediate layer and consists of a soft-magnetic material, e.g. of a nickel-iron alloy. In another embodiment, the core consists of a magnetically non-conducting but electrically conducting metallic inner conductor, which has a high reluctance and consists, e.g., of a beryllium-copper alloy and on which first the hard magnetic layer, then the intermediate layer and finally the soft magnetic layer have been deposited. The pulses generated by such a bistable magnetic element will be smaller than those generated by a Wiegand wire.
Another bistable magnetic element consisting of two layers has been disclosed in EP-A2-0 085 140 and is similar to the bistable magnetic element known from German patent specification No. 25 14 131 in that a hard magnetic core is surrounded by a soft magnetic core which differes in composition from the core. But different from the bistable magnetic element known from German patent specification No. 25 14 131 that layer has been twisted.
Such bistable magnetic elements may be used instead of Wiegand wires within the scope of the invention even if they have no homogeneous composition.
Magnetic field sensors comprising Wiegand wires consist in the simplest case of a Wiegand wire and an electric winding, called detector winding, which is wound around the Wiegand wire. Such magnetic field sensor affords the advantage that it can be excited to deliver Wiegand pulses even when the sensor is not supplied with electric power. The energy required for the pulses is taken from the magnetic field, which acts on the Wiegand wire and reverses the direction of magnetic flux in the wire when the requirements for an asymmetrical or symmetrical excitation described hereinbefore have been met. But that advantage afforded by such magnetic field sensor cannot be fully utilized unless the generated pulses can be transmitted over a certain distance without a need for a supply of additional energy because the energy required for the transmission is derived from the pulse generated by the magnetic field sensor. Specifically, it would be desirable to transmit the pulse generated by the magnetic field sensor wireless and not referred to a potential, e.g., by means of a radiofrequency transmitter which is directly energized and controlled by the magnetic field sensor, or by an optical fiber link having at the transmitting end a light-emitting diode which is energized directly by the magnetic field sensor.
Investigations have shown that the energy of the pulses which can be generated by a Wiegand wire or similar bistable magnetic element is insufficient in most cases for such applications. For this reason it has already been attempted to bundle a plurality of Wiegand wires and to surround the resulting bundle with a common detector winding so that the pulse energy delivered by the magnetic field sensor will be multiplied. But it has been found that a magnetic field sensor comprising a bundle of Wiegand wires will generate a strongly jittering burst rather than the desired single pulse having a high energy.
A bundle of Wiegand wires does not induce a common single pulse in a surrounding electrical winding because owing to inevitable slight variations in material composition the direction of magnetic flux in different Wiegand wires is reversed at slightly different field strengths, i.e., at different times. This effect is intensified by the fact that in a bundle of Wiegand wires the magnetic field of that Wiegand wire in which the direction of magnetic flux is reversed first will weaken the external magnetic field used to apply the triggering field strength for the Wiegand wires so that adjacent Wiegand wires having such a material composition that they require a somewhat higher triggering field strength than the Wiegand wire which has been triggered first will be triggered even later in the bundle than if they were independent Wiegand wires.